Add Group, Limit, Map, Sort functions

Signed-off-by: davidarendsen <davidarendsen@hey.com>

tests/Function/LimitFunctionTest.php

@@ -0,0 +1,18 @@
+<?php
+declare(strict_types=1);
+
+use Arendsen\FluxQueryBuilder\Function\Limit;
+use PHPUnit\Framework\TestCase;
+
+final class LimitFunctionTest extends TestCase {
+
+    public function testSimpleLimit()
+    {
+        $expression = new Limit(1);
+
+        $query = '|> limit(n:1) ';
+
+        $this->assertEquals($query, $expression->__toString());
+    }
+
+}